The invention relates to a process for controlling the degree of attrition of a sealing screw conveying a plasticized material, in particular synthetic plastic material, against a resistance body, which screw is bearingly supported within a screw housing between two screw sections located axially spaced apart from each other, the pitch of the threads of the sealing screw being directed opposite to that of the two screw sections, wherein the housings of the two screw sections are interconnected by a connecting channel leading the material conveyed by the two screw sections, a station for processing the material flowing within the connection channel, in particular a filter station, being connected to the connection channel, and wherein the resistance body is disposed between the sealing screw and that place at which the connection channel branches off the housing of that screw section through which the material flows at first. Further, the invention relates to an apparatus for performing such a process.
All screws bearingly supported within a screw housing and - although to a less degree - their housings are subject to wear, even when provided with best armoring, in particular screws conveying a material, containing abrasive components, for instance more or less soiled, mainly thermoplastic, synthetic plastic material of the recycling industry. With screws conveying against an extruder head, wear leads to the disadvantage that the pressure in front of the extruder head required for a perfect extrusion of the material cannot more reliably built up since the material slides back in an uncontrollable manner through the periphery gaps between screw and housing, which gaps occur due to the wear. Particularly undesirable is the wear with sealing screws which in the initially described manner are interposed between screw sections. Within this, the sealing screw is bridged by the connection channel forming a by-pass of the sealing screw. A wear in the region of the sealing screw, what occurs overtime, can be observed by gradual soiling of the material supplied to further processing, in particular to an extruder head, so that also the final product, as a rule a granulate obtained at the extruder head, is soiled. This causes complaints of the firms processing the final granulate. Soiled granulate has a less value and, in addition thereto, the granulate production of weeks must possibly be subjected to a second thermic filter processing, this; however, has the disadvantage that the second heat treatment requires unnecessary power consumption and that the molecular chains of the synthetic plastic material are adversely affected.
In order to avoid these difficulties, the sealing screw must be dismounted in regular time intervals and the housing as well as the screw must be checked in view of wear or attrition. However, this work - as experience shows - is hardly ever made, because it is time consuming and must be done by experienced people, because the wear in the region of the sealing screw can occur on this screw itself and this not only in view of the outer diameter of the screw threads, but also in view of the core diameter of the sealing screw, as well as on the cylindrical screw housing.
The invention has at its object to provide a process by which the control of the degree of wear of a sealing screw is possible without any problem, so that it can be seen at any time to what degree the wear of the screw or also its housing has been increased, and this also during operation. The invention solves this task by the features that the pressure of the material conveyed by the screw section that is at first passed by the material and the pressure of the material conveyed by the sealing screw are measured, in front of the resistance body, respectively, and that the difference of the two pressures is used for indicating the degree of attrition. In this connection, the invention starts from the perception that the pressure onto the material conveyed by the screw in direction of conveyance of the screw, therefore towards the resistance body, always increases as long as the screw works unobjectionably. If, therefore, the pressure at that place of measurement which is reached by the conveyed material at first, is designated by P1 and the measured pressure at the other place is designated by P2, then the function of the screw is unobjectionable if P2 is greater than p1. If both pressures P1 and P2 equalize, this so to be understood that the attrition has already been much increased, what, as a rule, means already a critical limit. If P2 becomes smaller than P1, then the function of the screw is no longer sufficient. When normally operating, namely, the perfect sealing screw conveys always a small portion of the material supplied via the connection channel to the second screw section back in direction towards the first screw section. If the function of the sealing screw is unobjectionable, the pressure of the material at that place of measurement which is located nearer to the sealing screw, is always greater than the pressure of the material at the other place of measurement. If both pressures become equal, this means that the sealing screw works neutrally, that is, no synthetic plastic material melt is conveyed by the sealing screw towards the first screw section. If the melt pressure at the place of measurement neighboring the sealing screw is smaller than at the other place of measurement, this means that, for example, the sealing section formed by the sealing screw is passed by unfiltered melt in direction towards the extruder head or towards an other outlet opening. The sealing function of the sealing screw, therefore, is then no more given to a sufficient degree and unfinished, in particular unfiltered, portions of the material will occur in the final product because the sealing screw then forms an undesired by-pass for the connection channel leading to the treatment station of the conveyed material, in particular to the filter.
xe2x80x9cScrew threadsxe2x80x9din this connection are to be understood by the volutions of the screw.
For comparison measurements of synthetic plastics material it is known to measure the pressure of the material at two places spaced apart from each other in a tube section which follows a screw section and has a reduced diameter. Since the diameter of this tube section is constant, the pressure at the location which is at first reached by the synthetic plastic material, is greater than at the other location, caused by the wall friction of the plastic material at the inner wall of the tube section. Further, no one of the two measurements is made within the region of the screw threads or the screw housing, and the measuring results obtained are only used for determining the properties of the synthetic plastic material and not for controlling the degree of wear of a screw.
The inventive apparatus for performing the process is characterized by the fact that two screw sections axially spaced apart from each other are interconnected for common rotation by a sealing screw, that an opening intersects the wall of the housing of the screw section that is at first passed by the material in the region of its screw threads or in an area immediately neighboring thereto, and a further opening intersects the wall of this housing or of the housing of the sealing screw, the two openings being spaced apart from each other, measured in axial direction of the screw sections, wherein the resistance body is disposed in the region of this spacing, and that to each one of the two openings a pressure sensor is connected. The indications of these pressure sensors are already sufficient for controlling the degree of wear of the screw or its housing in the above explained manner. However, it is more favorable, if transducers are connected to the pressure sensors, which at their output deliver an electrical signal to a comparison circuit, to the output of which an indication instrument is connected. This comparison circuit controls continuously the pressure difference of P1 and P2 and indicates it by means of the indication instrument. This instrument, in a manner known per se, can be so constructed that it produces a warning signal as soon as the pressure difference decreases a predetermined value. This warning signal may be of acoustic or visual kind and, if desired, may be immediately used for stopping the plant.
It is particularly favorable, if according to a further embodiment, at least one of the two openings in the wall of the screw housing branches off an annular channel neighboring the screw threads of the screw section or of the sealing screw. This has the disadvantage that the measuring result or the pressure sensor connected to the respective opening is free of impulses.
According to a further embodiment, the resistance body may be formed by a baffle body which is disposed between the two openings and dams up the supplied material and is disposed on a shaft connecting the screw sections with the sealing screw. This results in a simple construction inasmuch this baffle body may have a plurality of constructions. The simpliest embodiment consists in that the baffle body is a disk which confines a narrow annular gap between its periphery and the wall of the housing of the sealing screw; a further embodiment of this construction may consist in that at least one annular groove is provided on the periphery of the disk. This annular groove causes turbulences in the plasticized material passing the disk, what increases the resistance for this material. Alternatively, baffle body may be a screw thread section, the screw threads of which have the same conveyance direction as the screw threads of the sealing screws, but are finer. These fine screw threads cannot convey the entire material supplied by the comparatively coarser screw threads of the sealing screw. The difference in the amount of the material conveyed by the sealing screw, therefore, is xe2x80x9cshiftedxe2x80x9d over the fine screw threads, that means that it is pressed pass the fine screw threads, what causes an increase of pressure at the end of the sealing screw.